Just to clarify, the steps in the first paragraph will only hide things that have already been graded.  The "Hide grades" menu will not affect anything graded after that selection has been made.  The best way to do this is outlined in your second paragraph.  Before any grading is done, change the grade posting policy for the assignment to Manual, then do all of the grading, and use the Post option when ready for students to see the grades.

-Chris